I followed this four-turn pattern, similar to what a lot of others have been using for this fight:
Lionel, 2B, and Eliza use their elemental attacks to break the sentinel with a 75% chance of success. If it has its wind-lightning weakness, reset.
<aside> 💡 Note: There’s only about a 31.64% chance that the sentinel won’t have the wind and lightning weaknesses for all four rounds, so this may take a few attempts.
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Tikilen uses Chain Windspell. Because she’s the slowest of the warriors to act on this turn, she’ll chip away the Trial Warrior’s shield, assuming the other three were able to break the sentinel.
Kouren and Tikilen max boost attack to shield shave. If your team needs to be healed, have Eliza do so and use either Lionel or Olberic to attack. This will leave you vulnerable to some RNG if the Trial Warrior hits the same person too many times. If your team doesn’t need to be healed, Eliza can attack, and Olberic can max boost Defend.
Have Lionel break the Trial Warrior. If the Trial Warrior has more than four shields left, Lars can max boost Brutal Blazing Blade or Trifold Slash to help break (or you can use any other warrior that’s fast enough). Your other strong attackers should unload some damage and debuff if possible (e.g. Crumbling Cut with Fiore).
If this is your last cycle (turn 15 of the run), make sure you use the strongest moves you can since you’ll need to finish off the Trial Warrior.